By Brenda Goodman HealthDay Reporter ...TUESDAY April 30 (HealthDay News) -- Preteen girls may get the same i...HPV is a sexually transmitted infection that causes cervical cancer t...The HPV vaccine approved by the U.S. Food and Drug Administration in ...Current government guidelines advise parents to vaccinate boys and gir...

TUESDAY, April 30 (HealthDay News) -- Preteen girls may get the same immune response against human papillomavirus (HPV) with two doses of vaccine as young women get with the full three-shot series, a new study suggests.

HPV is a sexually transmitted infection that causes cervical cancer, the second biggest cancer killer in women around the world.

The HPV vaccine, approved by the U.S. Food and Drug Administration in 2006, is given in a three-shot series. The first and second doses are given one to two months apart, followed by a third dose six months later.

Current government guidelines advise parents to vaccinate boys and girls against HPV starting at age 11. That's partly to ensure they're protected against HPV before their first sexual encounter, but it's also to take advantage of biology.

"The immune response wanes with age. So the younger you are, the better immune response you have," said Dr. Jessica Kahn, a pediatrician at Cincinnati Children's Hospital Medical Center in Ohio. Kahn wrote an editorial on the study, but she was not involved in the research.

Although HPV vaccines are considered highly effective, fewer than one-third of U.S. teens receive all three doses, according to the U.S. Centers for Disease Control and Prevention's National Immunization Survey of Teens.

Doctors have wondered if a different dosing schedule might save money and time while still providing the protection of the three-dose regimen.

To find out, researchers in Canada, where vaccines are delivered through school-based programs, randomly assigned 520 girls aged 9 to 13 to receive either two or three doses of the Gardasil vaccine, which protects against four HPV strains. The girls who got two doses got their shots six months apart. The girls who got three doses got the vaccines on the regular schedule.

The younger girls were compared to 310 young women aged 16 to 26 who got three doses of the vaccine on the regular schedule.

Researchers took blood samples to measure the number of antibodies made against each viral strain. They continued to take blood samples over time to see how long the antibody response might last.

The younger girls who got two doses of vaccine appeared to make at least as many antibodies against the HPV strains as the teens and young women did on the three-dose regimen. And their protection appeared to last just as long, up to three years after they started their shots, according to the study, published April 30 in the Journal of the American Medical Association.

The researchers cautioned that while the antibody response to two doses of the vaccine looks promising, it doesn't prove that the shorter course actually protected against viral infections or cancers. A much longer study is under way to test that.

But study author Dr. Simon Dobson, a clinical associate professor with the vaccine evaluation center at the University of British Columbia, said the results are encouraging because they suggest that younger girls could get two doses to prime their immune systems against HPV, and then get a later dose to boost that response closer to the time they might become sexually active.

"It raises the possibility that you could give two doses early in preadolescence, and then wait and give the third dose later in adolescence when the girls are going to be closer to the time when they're most likely to need protection against HPV," Dobson said.

Two doses would also save money, a key consideration especially in countries where resources are scarce. According to the CDC, the Gardasil vaccine cost about $130 a dose in 2012.

Until more is known, however, experts say girls should get all three doses.

Kahn said doctors don't know if two doses will be enough to fully protect teenage girls.

"We need data on girls older than 13. Because even though the vaccine is recommended to be targeted to 11- to 12-year-olds, girls in the 13- to 17-year-old age range are actually most likely to be vaccinated. We would need to see data on immunogenicity or immune response in the 13- to 17-year-old age range," said Kahn.

And although protection from the vaccine appears to last a long time, doctors still aren't sure how long, or whether three doses might protect for longer than two.

"The data are encouraging, but it's really too soon to change recommendations based on the results of this one study," Kahn says.

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